Pages

Saturday, October 24, 2015

A European Spacecraft to Accompany NASA’s Europa Spacecraft?

Last year, NASA’s managers invited the European Space Agency (ESA) to
propose a small spacecraft to explore the Jovian system.The small craft would be carried to Jupiter
by NASA’s own, large Europa multi-flyby spacecraft.This daughter mission could add to the
exploration of Europa or study another target within the Jovian system.

ESA has recently posted the results of studies for two possible spacecraft
that might be carried by NASA’s Europa spacecraft to the Jupiter system. One would land on Europa and the other would
fly by the volcanic moon Io.While these
were concept studies and not an actual proposal from ESA to NASA, they give an
idea of the possible capabilities and limitations on an ESA contribution.

In the coming year, Europe’s scientists can make actual proposals for
spacecraft to be added to NASA’s mission.They can do so through ESA’s competition to select it’s fifth medium sized
(~550M Euros or somewhat more in dollars at the current exchange rate) science
mission.Proposals for the Europa
mission will be pitted against other planetary and astrophysics missions.However, because NASA would cover the costs
of launch and delivery to the Jupiter system, proposals for the Europa mission
could have a leg up in the competition.

A small ESA spacecraft would need to find a scientific angle not
already taken by its larger cousins. Next year, NASA’s Juno spacecraft will study the Jupiter itself from
just a few thousand kilometers above the top of the cloud deck.ESA’s JUICE spacecraft will arrive in the
late 2020s to study Jupiter from afar, flyby Europa and Callisto multiple
times, and then orbit Ganymede.NASA
Europa Mission (apparently no longer called the Europa Clipper) will flyby
Europa 45 times as well as flyby Ganymede and Callisto.These missions will carry suites of extensive
and highly capable instruments.

Under NASA’s proposal, the American space agency is reserving space and
250 kg of mass to host the European spacecraft.(NASA is also separately reserving mass for the equipment to connect the
two spacecraft.)

So if you had a ride to the Jupiter system for 250 kg, what could you do with it?

Two obvious possibilities were mentioned at the time that the offer was
announced: build a small lander for Europa or a small spacecraft that would fly
through any plumes erupting from Europa’s surface.The ESA team looked at both.

For a lander, the European study group considered a type of hard lander
known as a penetrator.Shaped roughly
like a cannon shell, penetrators smack into the surface and then travel a few
meters into it before coming to a stop.(Think of a bullet shot into the ground.The friction between the bullet’s surface with the soil slows and
eventually stops the bullet.)

Concept illustration of a Europa
penetrator (blue) embedded in the icy surface.An after body (green) remains on the surface with the antenna to
communicate with NASA’s Europa mission spacecraft.A cable would connect the two sections.Credit: ESA

Penetrators have been used on the Earth to deploy sensors from planes into
remote locations such as ice shelves.Penetrators
have the advantage of not requiring expensive landing systems – the penetration
into the surface supplies the braking.A
unique advantage on Europa is that once buried, the surrounding ice protects
the probe from the radiation fields around Europa.

The concept studied would use the penetrator to deliver two sets of
instruments in to the ice.The first set
would study the chemistry of the ice and materials within it.A habitability package would use chemical
reactions to look for conditions such as pH consistent with possible life, a
mass spectrometer would analysis the composition, and a microscope would image
the sample.A seismometer would record
Europa-quakes to study the level of activity within the icy crust and to gather
clues about its structure.

The penetrator with its deployment
stage.Credit: ESA

The penetrator would be delivered by a deployment stage that would
essentially be a small spacecraft with a substantial retrorocket.The main NASA spacecraft would target the
daughter craft to pass just 35 km above the landing zone.Just before that distance, the European spacecraft
would fire its rocket, reducing its speed to zero relative to the Europan
surface below.The delivery spacecraft
with the attached penetrator then begin their free fall to the surface.The delivery craft would have 231 seconds to
reorient itself so that the penetrator points straight down and then release
it.NASA’s Europa spacecraft passing overhead
would have a short few minutes to listen for a radio confirmation that the
landing succeeded.

The penetrator deployment.SRM burn is the rocket burn that removes all
velocity between the penetrator and the surface of Europa.The diagram uses the older name ‘Clipper’ for
the NASA spacecraft.Credit: ESA

Approximately ten days later, the NASA spacecraft would return to
Europa and receive the data collected by the penetrator’s instruments.

While the penetrator concept is exciting, the devils are in the
details.Planetary penetrator missions
have been studied for many potential missions.Russia launched two on a mission intended for Mars but which never left
Earth orbit.NASA delivered two tiny
penetrators to Mars, but they were never heard from after they were released by
their carrier spacecraft.Japan spent
years developing penetrators for the moon but eventually cancelled the project
because of development problems.

A key problem with penetrators is that they need relatively flat landing
sites for successful landings.Europa’s
surface is covered in slopes and rough terrain.Also, Penetrators are built to tolerate high vertical velocities, but
any lateral velocity can destroy the payload inside.(Put another way, engineers can design for
high G’s in one direction, but it’s hard to design for all directions.)This means that the retro rocket must
successfully kill all but the smallest lateral movement so the penetrator moves
only vertically during its descent.

Another problem with penetrators is that the space inside is small and
any instruments must be built to withstand high impact forces.As a result, there’s usually significant instrument
development required.The penetrator
concept report states that the instruments to study the chemistry of the ice
are at a low state of technical readiness for use in a penetrator.

Issues such as these have kept penetrators as a great idea that has
never been matured enough to become a reliable tool for planetary exploration.

My take on the report descripting the penetrator concept is that
delivering a penetrator for Europa appears to be a high risk possibility both
for completing the development in time for a launch and for actual
delivery.Another significant problem is
that the concept craft would have a mass greater than 300 kg, well above the
250 kg NASA is offering.

The other concept studied by ESA’s engineers would be a daughter
spacecraft that would be a straightforward use of existing technologies.The original idea was for a small spacecraft
that could fly through plumes erupting from Europa.This idea seems to have lost its appeal.First, diligent searches have failed to
confirm the original observation of a possible plume (which was made at the
limits of detectability).Second, NASA’s
Europa spacecraft is highly capable with cutting edge instruments, and it could
fly through any plumes itself.

Between the Juno, JUICE, and Europa missions, almost all of the Jupiter
system is already targeted for detailed study.An exception, though, is the extremely volcanic moon Io that sits deep
within Jupiter’s radiation field.That
moon became the target for the second study.

In this concept, NASA’s craft would release the European orbiter
shortly after the two jointly enter Jovian orbit.The ESA craft then would fire its own engine
to lower the perijove of its orbit to encounter Io at least twice.

The Io flyby spacecraft would be so small
that this top view engineering diagram uses millimeters for its length
units.The triangular main spacecraft
body would be below the center main antenna dish.Credit ESA.

The Io flyby spacecraft would carry four instruments.A multi-color camera would image the surface
at resolutions ranging from 2.2 km to 18 m per pixel, a thermal mapper would
identify and measure the temperature of hotspots at resolutions ranging from 30
km to 50 m per pixel, a mass spectrometer would measure the composition of ions
and particles ejected from Io, and a magnetometer would study the magnetic
field around Io.

The trajectory for each of the two Io
flybys (second flyby shown here) would be chosen so that the ground tracks pass
over several areas of known volcanic activity so that these areas could be
studied in high resolution.The first
flyby would pass 500 km above the surface at closest approach, the second flyby
100 km.Credit ESA

Jupiter’s radiation is strongest in the plane of the equator where its
major moons, including Io, orbit.Since
the ESA craft would be released from the NASA craft in an equatorial orbit, the
ESA mission would receive the full radiation baking.ESA’s spacecraft would have to traverse this
radiation on both the inbound and outbound legs of its passes.(A larger, fully dedicated Io mission such as
the proposed Io Volcano Observer proposed for NASA’s Discovery program, would
use a polar Jovian orbit instead, limiting its radiation exposure.)The tiny ESA spacecraft potentially could
perform more than two flybys if the harsh radiation close to Jupiter degrades
the craft’s electronics more slowly than expected.

The Io spacecraft study report does suggest that if the idea of an Io
spacecraft is pursued, that the option of releasing it before the Jupiter orbit
insertion burn is done should be considered.That way, the Io spacecraft could do its own insertion burn and enter a
Jovian polar orbit to reduce radiation exposure.

The Io flyby concept studies would come in on the heavy side by missing
the 250 kg target mass by 17 kg.To get
that close, the concept design had accept a “more risky operational scheme”,
that is, reduce backup systems and capabilities to minimize weight.

Either of these missions would be an exciting addition to the already
planned JUICE and Europa missions.The
Io flyby craft seems to be less risky both to design and to fly.

When NASA first announced that it would offer room and mass for an ESA
probe, ESA’s managers said they would let scientific teams propose which
concepts would be considered as part of the next Medium science mission
competition.These two proposals are
proof of concept studies that the proposing teams can use to inform their
proposals.We may be seeing even more
interesting proposals from the science teams.I can think of several alternative probes, and I’m sure the
professionals can think of even more creative ones.

If ESA does contribute a small probe to NASA’s mission, the exploration
of the Jupiter system may be more interesting than it already promises to be.

3 comments:

Always great news on potential planetary probe ideas, but my hope is that this mission does not get too bloated to where it is delayed (we really need this to fly on the SLS if at all possible), or sub ject it to too fats too soon and it never reaches its target....which would eliminate any future flagship missions for many years to come. Also, curious as you are the first to report it will no longer be called the Europa Clipper!!

What's wrong with "Europa Clipper"? I think that was a decent name, altough not so evocative as "Galileo"... looking forward for the official name ("Europa Multiple-Flyby Mission" is not the definitive name, right?)Anyway... glad to see that the possibility of a sub-probe to Jupiter has been explorated by ESA; I still hope in a lander; I read about an idea of a micro-lander that make use of inflatables for the touchdown, as part of the aborted Jupiter Icy Moons Orbiter (search the item on wikipedia and you can see an image of this concept). Less enthusiast for the Io idea... I don't see how such a tiny spacecraft can include a radiation protection AND a decent payload. For me isn't worth of the effort.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.